"Fractional Fermi sea": Innsbruck creates new quantum state from caesium atoms

A team led by Hanns-Christoph Nägerl of the University of Innsbruck and theorist Alvise Bastianello (CNRS and Université Paris-Dauphine) studied ultracold caesium atoms confined to one dimension. The researchers repeatedly cycled the interactions between the atoms between strong repulsion and strong attraction. This drove the atoms from their ground state into a highly excited state without losing their underlying order. The theoretical work appears in Physical Review Letters.

The state shows wave-like density ripples and an unusual way in which particle correlations weaken with distance. These signatures differ from the predictions of the long-established Tomonaga-Luttinger liquid theory for one-dimensional quantum systems. A companion experimental study is still undergoing peer review.

"Instead of simply heating the system, the interaction cycle reorganizes the atoms into a new many-body state," says lead author Yi Zeng. "This state is highly excited, but it is not random," adds Nägerl. He tentatively suggests calling the collective excitations "super-Fermions". The study names no direct application – it is fundamental research using quantum simulators.